The permanent mammalian myogenic cell lines, L6 and L8, can be induced to undergo modulation between myogenic, chondrogenic, and fibrogenic differentiated cell phenotypes. These cells, therefore, provide a well defined biological system in which to study the control of specific gene expression during these developmental programs. We propose to identify and quantitate the interacting levels, transcriptional and post-transcriptional, of gene regulation that are involved in cell differentiation. These experiments will utilize cDNA clones and genomic DNA clones specific for sm-tropomyosin, C1 collagen, alpha-actin, embryonic myosin light chain 1, and acetylcholine receptor-alpha subunit mRNAs. The expressions of sm-tropomyosin and C1 collagen are repressed during myogenic differentiation, while the three other genes appear to be activated coordinately. The cloned DNA sequences will be used to determine the rates of transcription, the kinetics of formation from nuclear precursors, nucleo-cytoplasmic transport, and cytoplasmic turnover of these mRNAs. A detailed structural analysis of these transcriptional units will be related to common features in the metabolism of their gene products. The combination of metabolic and structural approaches will provide a basis for molecular studies on myogenic cell mutants and neuromuscular morphogenesis, and will contribute to an understanding of the regulation of gene expression during development.